fix(core): initialize route to avoid race (500)

[StarlightIbuki]

Summary

The init router creation could be interrupted and we assume it will always be a non-nil value after the init_worker phase, which may cause 500 to proxy requests.

Let's create an empty router before creating the initial router.

As the issue occurs at this version I omitted the changelog.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix KAG-5815